Electrical drive systems with a variable speed may be used to run a machinery such as a pump, a fan or a compressor at different speeds. The machinery is usually connected via a shaft with the drive system. The machinery can consist of a single component or of multiple components along the drive shaft. The most common drive systems comprise a drive in the form of a variable speed AC or DC electric motor. Varying the speed of the machinery has several advantages compared to fixed-speed operation, the most prominent one being the ability to save energy by running the machinery at lower speeds.
During control of such a drive system, it is often assumed that the rotational speed is equal over the length of the drive shaft. Additionally, it is often assumed that the rotational speed derived from the electric signals of the electric drive is equal to the rotational speed of the machinery. Similarly, the torque at the air gap of the electric drive is assumed to equal the torque applied to the machinery.
These assumptions may be valid as long as the drive shaft has no distinct natural frequencies, or is run in a low dynamic fashion. The applications that have emerged in recent years however may have high dynamic demands such that those assumptions may not be valid and the dynamics of the drive shaft may have to be taken into account.
One example for such an application is the operation of compressor trains in the oil and gas industry, where electrical drive systems may replace commonly used gas turbines. In these applications, the drive shafts usually are long and relatively thin, resulting in a flexible, weakly damped structure with easily excitable natural frequencies. The commonly made assumption that the drive shaft is a single mass rotating with a certain speed may often not be valid in these applications. The torques and speeds at different points of the drive shaft may differ significantly from each other.
Furthermore, it may be a demand in these applications to control highly dynamic processes, in which the speed of or the torque applied to the machinery has to be varied according to the needs of the process. An example may be compressor station anti-surge control, where the torque applied to the compressor is varied to prevent surge events. These events may require immediate actions, and oscillations in the torque of the compressor may not be acceptable in the case of a surge event.
Additionally, in these control schemes, the controlled variables are preselected and cannot be changed. A selection of a different mechanical variable, such as the torque at the machinery, or a limitation of other variables, such as torsional strain at a coupling, is not part of the control scheme. However, the application of a drive may underlie different requirements in different industries. In one application, the angle of the machinery may be of importance, in the next the torque or the speed. These different situations are usually handled by applying different control schemes for different situations.
In “Model based predictive speed control of a drive system with torsional loads—a practical approach”, Power Electronic and Motion Control Conference, 2010, IEEE 2010, a model based predictive speed control of a drive system is disclosed and takes into account the torsional oscillations of long driveshafts in an electro-mechanical drive arrangement characterized by fast dynamics. Therefore, it discloses a method for controlling an electrical drive system and the electrical drive system comprising a drive mechanically interconnected with a machinery for processing an industrial process.
Furthermore, US 2012/0059642 A1 discloses a device and a method for determining position information of the rotor shaft of an electric machine based on at least one recorded input signal of the electric machine. The recorded input signal being supplied to a model of the electric machine. The position information of the rotor shaft is determined with the aid of the model, based on the supplied input signal, the model mapping nonlinear saturation effects of the electric machine.